1. Field of the Invention
The present invention relates to a novel group of particles used for air-filter inspection (test), and more particularly to, for example, a group of particles used for inspecting an air filter for use in clean rooms and an inspection method for air filters using such a group of particles.
2. Description of the Related Art
Elimination of dusts in the air is normally carried out by making the air pass through an air filter. In particular, in order to eliminate dusts from a clean room, an air filter with high performance, such as an high efficiency partyiulate air filter (HEPA), and a ultra low penetration air filter (ULPA), which can capture not less than 99.97% of fine particles having a particle size of 0.3 .mu.m, is used, and these fine particles inside the clean room are constantly eliminated so that a predetermined degree of cleanliness is maintained in the clean room. When there is any leakage in the above mentioned high-performance air filter, such as a leakage due to a pin hole in the filter material or a leakage in a connecting section between the air filter and the frame, it may exert a serious influence upon the degree of cleanliness of the clean room, therefore, all the high performance air filters to be used are actually subjected to an inspection for any leak or reduction in its performance.
Here, in the present specification, "inspection" includes various inspecting operations for any leakage in the air filter or for air-filter performances. These inspecting operations are conducted as follows:
While airflow is made to direct to an air filter, a group of particles used for filter inspection or an aerosol including such particles is supplied to the airflow at the upstream side of the filter, and the group of the particles leaked out of the filter is detected at the downstream side of the filter by using, for example, a particle measuring device.
In general, a group of particles of a liquid organic compound having a low vapor pressure, such as dioctyl phthalate (DOP), stearic acid, dioctyl sebacate (DOS) and paraffin oil are used as a group of particles for filter inspection as standardized in JIS Z 8901. The group of the above particles is generated by an aerosol generator using a vapor condensing method or a pressurized atomization method.
However, when a group of these particles is used to inspect the above mentioned high performance air filters, a problem arises in which, in spite of its low vapor pressure, the organic compound vaporizes from a small quantity of particles that were captured by the filter material of the air filter and adhere thereto. Therefore, when an air filter which has been subjected to the inspection by using such a group of particles is installed in a clean room for semiconductors, the adhering organic compound may volatilize and scatter and its vapor may outflow from the air filter to the downstream side, and adheres onto, for example, silicon wafers, thereby adverse effects on the wettability and electrical characteristics of the silicon wafers, and consequently a reduction in the yield of the semiconductors may be caused. Additionally, with respect to DOP, its possibility of causing cancers has been conventionally pointed out, and recently, its harmfulness as an endocrine disrupter is pointed out.
For these reasons, a group of particles, made of solid substances which is inherently free from evaporation and outflowing, has been researched and developed.
For example, a method in which dusts in the air are used has been known; however, since the size, shape and concentration, for example, of dust particles are not uniform, it is difficult to carry out inspections stably with high reproducibility. Moreover, when the concentration of dusts in the air is low, it takes a long time to carry out the inspection, and there is a possibility such a method fails to find out leakage. Furthermore, since dusts in the air may contain heavy metals, etc., it is not preferable to allow them to adhere to the air filter.
Moreover, Japanese Patent Kokai Publication No. 317690/1993 (Tokukaihei 5-317690) discloses a aggregate particles, as a group of particle for filter inspection, which is obtained through atomizing a suspension of polystyrene latex (PSL) particles that are solid particles having a known particle size and a globular shape, pulverizing the generated liquid droplets and drying the droplets. However, since the PSL particles easily coagulate in a dispersant, it is difficult to prepare a suspension with a high concentration. Therefore, in order to obtain a particle concentration of the PSL particles necessary to function as particles used for air filter inspection, a device having a complicated and special mechanism is required; consequently, this method fails to easily provide aerosol having a sufficient concentration.
Furthermore, Japanese Patent Kokai Publication No. 136437/1996 (Tokukaihei 8-136437) discloses an inspection method for air filters in which: a suspension of silica particles having particle size ranging from 0.055 to 0.18 .mu.m is atomized and dried, and the resulting silica particles having a particle size of not less than 0.1 .mu.m are used. However, the silica particles obtained in this method have extremely deviated shapes from a globular shape in particles having a particle size in the vicinity of 0.1 .mu.m to be used for inspection, and these particles have uneven surfaces with many protrusions and recesses (hereinafter, referred to as "irregular shape"). And the following problems may be caused.
With respect to a particle counter used for filter inspection, a light-scattering-type automatic particle counter (LPC) is generally used since it allows a simple measuring operation and continuous automatic driving, and provides the results of a measurement at once. In the measurement using the LPC, the particle size is measured by detecting the intensity of scattered light that is caused by scattering of radiated laser beam by the particles to be measured, and the particle size is normally obtained as a size corresponding to the true globe of a PSL particle. When the particles have irregular shapes, the scattered light varies depending on changes in the orientation of the particles to the laser light beam; consequently, there may be a great error in the obtained particle size. Therefore, it is difficult to measure the particle size stably with high reproducibility and also to correct the obtained particle size.
Moreover, when the particles have irregular shapes, fluid resistance that the particles undergo varies depending on the orientation of the particles to the fluid flow; therefore, the particle size that is obtained by using the LPC is not always consistent with the particle size that is subjected to fluid resistance in a gas flow (hereinafter, referred to as "aerodynamic particle size"). It is known through experiment that an aggregate in a lump shape with constituent particles of around 10 may undergo a fluid resistance 1.23 times larger than a globe having substantially the same size. Even though the difference between these two particle sizes is in the order of 0.01 .mu.m, it will give a serious adverse effect on the reliability of the filter inspection; and in particular, in the case of high performance air filters such as HEPA and ULPA, the capturing efficiencies of the filter may differ by an order of magnitude in such an error range.